The present invention relates in general to master cylinders used as generators of hydraulic pressure in motor-vehicle braking systems.
More specifically, the invention relates to a hydraulic master cylinder of the type comprising: a body pierced with an axial bore of given diameter delimited by an at least partially cylindrical wall, a first end of which forms an opening that is partially shut off by a guide ring fixed to the body, and a second end of which is closed by an end wall, this body having a drilling forming an inlet for a hydraulic fluid at low pressure; a piston of cylindrical shape passing through the guide ring, mounted to slide axially in the bore between a position of rest and an extreme actuating position, and delimiting within the bore a variable-volume pressure chamber; an upstream sealing collar secured to the body and gripping the piston in a sealed manner; a downstream sealing collar secured to the piston and co-operating with this piston to shut off the pressure chamber in a sealed manner, the downstream collar being closer to the end wall of the body than is the upstream collar; at least a first spring applying to the piston a first axial force which urges it towards its position of rest; and a radial filling duct connecting the fluid inlet to the pressure chamber when the piston is in its position of rest.
Master cylinders of this type are well known in the prior art, as illustrated, for example in U.S. Pat. No. 5,154,107.
In accordance with the teachings of this patent, the filling duct consists of a radial drilling which, insofar as it has a circular cross section, has to have a relatively large diameter so that it can correctly fulfil its intended function.
Now, this constraint on the one hand has the effect of increasing the axial length of the master cylinder, and on the other hand, has the effect of increasing the fatigue of the downstream collar at the point where this collar passes across the filling duct.
The present invention falls within this context and aims to escape from the aforementioned constraint and the drawbacks it entails.
To this end, the master cylinder according to the invention, which in other respects is in accordance with the definition given in the above preamble, is essentially characterized in that an axially sliding ring surrounding the piston and of the same diameter as the bore is placed in the body between the guide ring and the pressure chamber, in that a second spring applies to the sliding ring a second axial force, lower in intensity than the first force, pushing the sliding ring back away from the guide ring, in that the piston and the sliding ring have respective stops which co-operate to allow the sliding ring to be driven axially by the piston when the piston returns to its position of rest, and in that the filling duct comprises an annular space of variable axial length defined between a front edge of the sliding ring and a flared portion of the axial bore, adjacent to this front edge.